||Abstract of \"Stock assessments and biological studies for swordfish, billfishes, and southern bluefin tuna in the three Oceans\":
1. \"Studies on CPUE standardization and stock status for Pacific blue marlin and North Pacific and Atlantic swordfish\" :
Fishery data and catch information of the Taiwanese longline fleets in the Pacific and generalized additive model (GAM) to examine the trends of relative abundance index. All the effects considered in the CPUE standardization including year, month, latitude, longitude, and the interactions were statistically significant (P ＜ 0.05). Standardized CPUEs of the Pacific blue marlin showed an increasing trend since 2000, but decreased slightly from 2005 and relatively stabilized in recent two years. The standardized CPUE of North Pacific swordfish decreased during 2002-2005, but increased since 2005. In both the South and North Atlantic Oceans, the standardized CPUE of swordfish sharply dropped in the late 1990s, and then stabilized from 2000 with higher values appeared in recent two years. The results were insensitive to the inclusion of gear configuration as explanatory variables in the model for all analyses of CPUE standardizations. The stock status of Pacific blue marlin and South Atlantic swordfish were assessed using an age-structured production model (ASPM). Results show that the status of Pacific blue marlin was fully exploited (S/SMSY = 1.00) and currently slightly overfishing (F/FMSY = 1.22). The swordfish in the South Atlantic Ocean was assessed to be overfished (S/SMSY = 0.89), but not overfishing (F/FMSY = 0.79). Assessment results for both species in this study suggest that these two stocks should be monitored closely.
2. \"Studies on stock status for billfishes in the Indian Ocean and southern bluefin tuna\" :
The objective of this study is to carry out the CPUE standardizations and preliminary stock assessments for striped marlin and blue marlin in the Indian Ocean. Standardized CPUEs of striped marlin reveals substantial decline since 1980. Standardized CPUE of blue marlin deceased since mid-1980s, revealed increasing trends during 1990s but they gradually decreased for recent years. Based on the results of stock assessments, the stock status of striped marlin might be very pessimistic, current (2011) fishing mortality might be higher than MSY level and current biomass might be lower than MSY level. The stock status of blue marlin might be relatively optimistic. However, different CPUE series might result in significantly diverse assessment results and thus current assessment results are very uncertain. In addition, this study also carried out CPUE standardization for southern bluefin tuna. The CPUE standardization is performed based on the sub-area effect of either CCSBT statistical areas or Taiwanese SBT fishing ground. Standardized CPUEs generally reveal quite different trends for different area. It is apparent that the CCSBT statistical areas are not appropriate for Taiwanese SBT fleet since the main fishing grounds are separated into several parts by CCSBT statistical areas. It would be helpful to redefine CCSBT statistical areas for Taiwanese SBT fishery according to the temporal and spatial analysis of its fishing characters.
3. \"Age, growth and ecological research of southern bluefin tuna and Pacific bluefin tuna\" :
In this project, we trained scientific observers to collect otoliths of southern bluefin tuna (SBT, Thunnus maccoyii) on Taiwanese longline vessels in the Indian Ocean. The otoliths collected from SBT and Pacific bluefin tuna (PBT, Thunnus orientalis) were used for age estimation. In 2012, 33 SBT heads were collected in the central Indian Ocean and otoliths were removed in the lab. The estimated mean ages were 1.9 ± 0.7 years (range: 1-4 year). Another batch of SBT otolith were collected in the central Indian Ocean 2007 and these samples were aged this year. The mean age of year 2007 samples was 3.4 ± 0.9 years with a range from 2-5 years. The observers also collected otoliths from 168 PBT landed in domestic harbors. The estimated mean age for these PBT was 15.5 ± 3.9 years (rang: 6-26 years). The historical length data of the landed PBT measured by the observers showed gradually increasing from approximately 210 cm at year 2002 to 230 cm in recent years (2010-2013). The estimated mean age of the landed PBT also gradually increase from approximately 11 to 17 years. The decreasing recruitment of the immature PBT might be the main reason that cause the increasing trend of mean size and age of PBT landed in Taiwan although other possibilities were carefully discussed in this report.
4. \"Environmental effects on billfish CPUE in the Pacific\" :
The catch and effort data of swordfish and striped marlin for the Taiwanese tuna longline fishery in the Pacific Ocean were collected for 2001-2011. GAMs with environmental factors included are applied to standardize the CPUEs of swordfish and striped marlin. For swordfish, The GAMs results showed that sea level anomalies, meridional wind speed, SST, SOI, PDO index and zonal wind speed were important factors in explaining the variance in CPUE in the North Pacific, and chlorophyll a concentration, SST, sea level anomalies and wind speed were important in the South Pacific. For striped marlin, meridional wind speed, SST, PDO index and SOI were important factors in explaining the variance in CPUE in the North Pacific, and SST, chlorophyll a concentration, SOI, sea level anomalies and meridional wind speed were important in the South Pacific. These environmental factors related to ENSO and PDO revealed that swordfish and striped marlin are in a higher CPUE level as PDO index positive and SOI negative.